82 compute the product using the methods below. if a product is undefined, explain why. a. the definition where ax is the linear combination of the columns of a using the corresponding entries in x as weights. b. the row-vector rule for computing ax. a. set up the linear combination of the columns of a using the corresponding entries in x as weights. select the correct choice below and, if necessary, fill in the answer boxes to complete your choice. o a. x121 + x2a2 + + xnan = (x7)a1 + (x2)a2 = ( + (type the terms of your expression in the same order as they appear in the original expression.) b. x1 a1 + x2a2++xnan = (x1) a, + (x2)a2 + (x3)az = ( + (type the terms of your expression in the same order as they appear in the original expression.) oc. the matrix-vector ax is not defined because the number of columns in matrix a does not match the number of entries in the vector x. od. the matrix-vector ax is not defined because the number of rows in matrix a does not match the number of entries in the vector x.